256 Proceedings of the Royal Irish Academy. 



move in the same orbit as the great comet III. 1862. The orbit of the 

 comet I. 1870, appeared at first very like that of the last-named comet ; 

 but my final determination of the orbit gave elements less like those of 

 the Perseid comet. All the same, it is very interesting that the two 

 comets sho^v their meteors almost on the same days, and the aphelia 

 being pretty near each other, the whole system of comets and meteors 

 form one of those " comet families," of which the researches of 

 IT. Hoek have found so many instances.* 



Subordinate radiants do not only appear in the case of the Perseids: 

 there are a good many such groups of radiants known. There are two 

 ways in which such neighbouiing radiants may have originated : 

 either a great accumulation of meteors moving in the track of a comet 

 may have been disturbed by the earth at some distance from it — great 

 enough to prevent the swarm from being scattered, and yet small 

 enough to make it move in a new orbit; — or the attraction of the earth, 

 on the single meteors, which pass us closely, may make the orbit have a 

 number of radiants instead of a single one. The curve, which a meteor 

 will describe round the earth, is a hyperbola, the asymptotes of which 

 are two Knes in the directions in which the meteor approaches and 

 leaves the sphere of attraction of the earth. When we pass through a 

 swarm of meteors, the earth is therefore perfectly enveloped by such 

 hyperbolas, along which the meteors will move in all dii'ections, 

 instead of going in parallel orbits, as they did before meeting the 

 earth. Once, therefore, a certain meteor has passed close by the earth, 

 it cannot, if the orbit is a closed curve, and we meet it a second time, 

 appear to come from exactly the same point in the heavens — in other 

 words, the radiant will be slightly difierent. According to Oppolzer, 

 the time of revolution of the great comet of 1862 is 121 years. If, 

 then, the Perseus-meteors are particles which gradually have been 

 separated fi'om the main body or nucleus of the comet by evaporation of 

 the parts nearest to the sun, and by the subsequent condensation of these 

 vapours into a number of small bodies, the main stream of these 

 would be surrounded by a number of scattered bodies, moving in orbits 

 more or less different fi'om that of the main stream, and these straying 

 meteors would, it appears, produce the phenomena of secondary radi- 

 ants. 



It does not, however, seem likely that the meteors, which radiate 

 from a point near ^ Persei, have originated in this way. As the radi- 

 ant is so near that of the comet I. 1870, the elements of which 

 are not a little different from those of the comet of 1862, it seems 

 probable that the two meteor-showers are independent of one anotehr, 

 each being composed of matter left behind by its mother-comet. But, 

 besides the meteors from near \ Persei, there are many other Circum- 

 Perseid meteor-showers, as amply shown by Mr. Denning (Brit. Ass. 

 "Report," 1878, p. 344); and it appears likely that at least some 

 of these have arisen from the perturbations caused by the earth, as 

 described above. 



^ " MontUy Xotices," E. A. S., sxy., xx-^-i. 



